This sounds to me like the lines aren't bleed properly or haven't been bled at all. Any time the calipers or brake cylinders or anything is pulled off or moves out of position the brake lines should be bled. A lot of "mechanics" will argue this and a lot of even service centers that do brakes almost never bleed them and that is not how to properly do brakes.
Improperly or not bleeding the brake lines is usually the cause of what you're describing as well as spongey brakes. The pedal should be hard as a rock, you should not be able to make the pedal touch the floor. If you are able to or they are spongey or locking up it's most likely that the fluid system needs to be bled.
How do you know they're overheating? Are they getting red hot? If they're not getting red hot it's unlikely they're overheating. Brakes do get hot, there's friction. It's friction that causes them to work, friction causes heat. It's normal for them to be hot to the touch after driving even a short distance but they shouldn't be glowing red - that's a problem.
How to bleed brakes:
Take the reservoir cap off and insure it's topped off with fluid. Replace the cap tightly then loosen it to allow air to escape. If you don't loosen it there's risk of blowing the seals out of the master cylinder or somewhere else in the brake system that will cause a leak. A leak will cause the brakes to not work.
Start at the wheel furthest from the master cylinder (reservoir). You will need another persons help. Locate the bleeder valve and crack it loose then tighten it back up. Have the other person pump the brake pedal until it gets hard, then have them hold it while you loosen the bleeder valve again, tighten the bleeder valve and tell your helper to pump and let go. Repeat this working your way from the furthest wheel from the reservoir to the closest and then do it again from the closest wheel to the furthest.
You want to keep doing it at each wheel until you get a solid stream of fluid. No air, no spitting or spraying. If it's spitting or spraying there's air. You want to remove all of the air pockets from the lines so it's fluid only, no air.
To prevent a mess of fluid on the ground and preserve fluid you can put back in the reservoir you can use a piece of plastic tubing and a jar or a plastic bottle to catch it.